This invention relates to an AC-DC power supply and, in particular, although not necessarily solely, to a power supply that is suited to provide the necessary DC supply for an electrolysis cell unit for the electrolysis of water to liberate hydrogen and oxygen gas. However, generally the invention is suited to any particular application where an AC mains supply is available but a DC supply is required.
The full wave rectification of a multi-phase AC power supply to produce a pseudo DC output is commonly known. For a 240 V, 50 Hz single phase mains power supply, the average DC voltage produced by a full wave (H-bridge rectifier) is 216 V. However, loads requiring a DC power supply typically require much lower voltages.
For electrolysis cell units such as the applicant's AQUAGAS 3 gas generator, a DC voltage in the order of 33 V is required. In order to accommodate this requirement, it has conventionally been the case that the AC mains supply is transformed to an appropriate lower level before rectification so that the desired output DC voltage is realised. The disadvantage with such a technique is that mains frequency transformers of a power rating above 10 kW become physically large and heavy due to magnetic circuit and leakage reactance considerations.
Another known technique to generate a desired DC supply from a fixed mains AC supply is to utilise controlled switching devices in the rectifier bridge. These may be devices such as power transistors, SCRs or GTOs. The average DC output can be adjusted by controlling the turn-on (and possibly also the turn-off) time for the switching devices. The disadvantage with this technique is that the output wave form is "lumpy" even if smoothing storage capacitors are used across the DC output. Certain loads are sensitive to time-varying changes in the DC voltage level. This includes such DC machines as electrolysis cell units. Some may be sufficiently sensitive as to reach the point of not operating satisfactorily with such an output wave form.
There are other problems associated with a known power supply of the type having a single phase AC supply, a step-down transformer and a controlled rectifier bridge circuit. These include high transformer inrush current on start-up and a susceptibility to phase imbalances if two phases of a nominal three phase supply are utilised rather than one phase and neutral, or all three phases.